The Thermodynamic Properties and Potential Energy of Solid Argon

Abstract

In an earlier paper an attempt was made to obtain the interatomic potential for a pair of argon atoms, using the specific heat data at low temperatures and a theory in which the Debye characteristic temperature Θ was related to the curvature of the interatomic potential energy curve. Difficulties in accounting for certain other experimental data were explained on the supposition that disorder sets in in the solid below the melting point. As there are certain objections to this point of view, a new calculation is presented herewith in which the Debye theory is assumed to hold throughout, and Θ and the potential energy of the solid are calculated independently of each other from the available experimental data on the specific heat at low temperatures, and on the equation of state at high pressures. A consistent picture of the thermodynamic properties of solid argon is thus obtained, but the interatomic potential curve for a pair of atoms differs greatly from that obtained by the method used previously. The interatomic potential curve found here shows extreme change in curvature near the minimum. This change in curvature is so great that, if the result is correct, the theory of small vibrations will not be applicable, and it will not be possible to relate Θ to the curvature, as was done in the earlier work. The significance of this conclusion for the theory of the solid state is discussed.